1. Technical Field
The present invention relates to a diffraction optical element, a lighting device, and a projector.
2. Related Art
A lighting device of a projector known in the art uses a metal halogen lamp, a halogen lamp, or other lamp. In recent years, however, the use of a semiconductor laser display (LD) is proposed for miniaturization of the lighting device and the projector (for example, see JP-A-11-64789).
A laser display apparatus disclosed in JP-A-11-64789 includes semiconductor lasers for emitting red, green and blue beams, a plurality of collimator lenses for collimating the laser beams emitted from the respective semiconductor lasers, a plurality of microlenses for equalizing intensity distribution of the lights collimated by the collimator lenses, and spatial light modulation devices for modulating the lights released from the respective microlenses.
The advantages of the laser beam sources included in this type of laser display apparatus are high color reproducibility, high luminance, and high contrast of image display, and immediate light emission from the laser beam sources as well as size reduction.
Also, a technology which converges entering light on liquid crystal cells by using a hologram (diffraction optical element) has been proposed (for example, see JP-A-9-68705).
According to the technology disclosed in JP-A-9-68705, however, there is a possibility that the hologram shifts in a direction crossing the center axis of the entering light at the time of assembly of the system including the hologram or due to change with time. More specifically, according to a hologram 200 assembled such that light enters an effective illumination area 201 as illustrated in FIG. 13A, the effective illumination area 201 of the hologram 200 gradually shifts its position with respect to a predetermined illumination area S as illustrated in FIGS. 13B and 13C.
In the condition shown in FIG. 13A, light enters the effective illumination area 201 of the hologram 200. Thus, the light released therefrom illuminates the entire illumination area S. However, in the condition shown in FIG. 13B, light enters the effective illumination area 201 of the hologram 200 with positional disagreement between the effective illumination area 201 of the hologram 200 and the predetermined illumination area S produced. In this case, the entering light illuminates an area out of the predetermined illumination area S. In the condition shown in FIG. 13C where the positional disagreement between the effective illumination area 201 of the hologram 200 and the predetermined illumination area S increases, entering light passes through an area out of the effective illumination area 201, and the predetermined illumination area S receives only light spot. When positional disagreement between the effective illumination area 201 of the hologram 200 and the predetermined illumination area S is produced as in these examples, light utilization efficiency decreases. As a result, unevenness of brightness develops, for example, and image quality thus lowers.